Electrically nonconductive materials may be coated with a metal by the utilization of inexpensive wet chemical techniques. The basic method employed comprises the use of a two-step sensitizing catalyzing procedure, using a tin salt and a palladium salt, followed by subsequent electroless plating of the desired metal on the nonconductive substrate. This basic idea of seeding a surface with palladium catalysts to initiate plating from an electroless plating solution is not new. This process is successfully used for printed wiring manufacture, decorative plating of plastic objects, and the like. U.S. Pat. No. 3,798,050 discloses a method of plating metal on substrates, and further discloses an improvement to that method. In that patent, effort is made to permit high speed assembly line manufacture of substrates, using a buffered solution to improve adherence of the metal to the substrate.
Certain characteristics have been observed when employing the chemical metallization techniques to various nonconductive substrate material. While the procedure works equally well on both polished and rough materials, adhesion to rough surfaces is generally better. Tests have been made using a tape test to determine adhesion to polished surfaces, however, and relatively good success has been achieved. In this tape test, application of a pressure sensitive tape to a metallized film, followed by rapid stripping of the tape, shows little, if any, metal adhering to the tape. Metal films formed by this plating process are extremely uniform and repeatable, thereby permitting manufacture of interchangeable quantities of relatively sophisticated designs.
An extremely small quantity of palladium metal is involved in the process, and, therefore, the expense is not substantial due to the use of this raw material. The palladium solution is extremely stable and may be used for an extensive period of time. The sensitized catalyzed surface prior to plating remains active for several days and is completely transparent and electrically nonconductive. The plated metal surface replicates the texture of the substrate. A polished glass substrate, for example, produces a mirror polished metal film.
While chemical metallization has found a variety of uses, one of the problems associated with the process is the difficulty to completely and totally cover the surface without defects or imperfections on the surface. It is known that metal will plate wherever the catalyst is present, although the mechanism for the plating is not precisely understood. While it may appear simple to ensure complete plating by merely ensuring complete coating with the sensitizing and catalyzing agents, the presence or absence of the sensitizing agents is difficult to ascertain. Particularly, this is true because the sensitizing catalyzing surface is completely transparent to visible light and is electrically nonconductive to thousands of megohms. Oftentimes, surfaces which are too hydrophobic will not wet adequately to permit formation of the sensitizing catalyzing agents on the surface. Impurities and contaminants will either mask or poison the sensitizing and catalyzing agents and will prevent a complete metallization free from defects and imperfections.
Accordingly, it is an object of this invention to provide a method for electrolessly plating metals on surfaces substantially free from defects or imperfections.
Another object of this invention is to provide a method for inspecting the sensitizing and catalyzing step in the electroless plating method to ensure complete catalyzation and therefore complete metallization of the surface being treated.
Yet another object of this invention is to improve the uniformity and adherence of the metal to the substrate.